The manufacture of cigarette filters from a continuous, multifilament filter tow generally involves processing steps which include separation of the individual filaments (i.e., "opening up" of the filter tow), the application of plasticizer and other additives to the opened up tow and the formation of a continuous filter rod from the plasticized filter tow. The uniformity and filtering characteristics of the resulting filter rod are largely determined by the effectiveness of these tow processing steps. Thus, there is a substantial amount of prior art which is directed to methods and apparatus for transforming filter tow into cigarette filters having predictable smoke filtration characteristics.
One of the prior art methods for converting filter tow into a filter rod involves passage of the tow through a circular jet where the tow is subjected to tension created by a gaseous fluid flowing through the circular jet in the direction of the advancing tow. U.S. patents disclosing methods and apparatus relating thereto include U.S. Pat. Nos. 3,099,594, 3,262,178, 3,262,181, 3,282,768 and 3,297,506.
In converting the filter tow into a filter rod, a suitable plasticizer is applied to the tow in such a way that the characteristics of the resulting filter rod are more or less uniform. In U.S. Pat. Nos. 3,099,594 and 3,282,768 (FIGS. 4 and 5) there is taught a method for injecting plasticizer into the moving stream of gaseous fluid so that application of plasticizer to the filter tow takes place in the turbulence created by the moving gaseous fluid. Distribution of plasticizer on the individual filaments of the filter tow in this method depends on the interaction between the gaseous fluid, the filter tow, the plasticizer and the circular jet in which the various components are temporarily confined. It has been found, however, that introduction of plasticizer into the stream of gaseous fluid does not result in uniform distribution of plasticizer on the filter tow. Rather, the plasticizer has a tendency to remain on the outer peripheral layers of filaments in the filter tow as it moves toward the tow exit end of the jet. Furthermore, a portion of the injected plasticizer emerges from the jet in the form of droplets if the quantity of injected plasticizer is not carefully controlled. Thus, the addition of plasticizer via injection into the moving stream of gaseous fluid has never provided completely satisfactory operation. Consequently, the commercial processing of cigarette filter tow has generally employed alternative methods involving the application of plasticizer to a flattened, spread band of tow by spray means or wick type applicators. Such alternative methods are disclosed, for example, in U.S. Pat. Nos. 3,157,536, 4,132,189 and 4,313,974.